Whereas propylene homopolymer, i.e., polypropylene (PP), and blends of PP with other polyolefins, such as ethylene homopolymers and copolymers, are highly useful, one widely recognized deficiency which limits their utility for certain applications is their relatively low melt strength due to the linearity of the PP. Efforts to increase melt strength have focused on the introduction of molecular features onto the polymer backbone which will increase entanglement between molecular chains. This has typically been accomplished by grafting various monomers onto the PP or by various procedures capable of modifying the chain structure, such as by the introduction of long-chain branching.
Sugitomo, et al., in their article entitled “Effect of Chain Structure on the Melt Rheology of Modified Polypropylene” (Journal of Applied Polymer Science, Vol. 73, 1493-1550 (1999)) disclose the use of electron irradiation and di-2-ethylhexyl peroxy dicarbonate to modify the molecular structure of linear PP and show the effect on rheological properties. Whereas treatment at high irradiation levels produced long-chain branching, they were unable to detect any long-chain branching upon treatment with the organic peroxide. They concluded that reaction with 1.2 weight percent peroxide only produced a small increase in molecular weight, as evidenced by an increase in melt flow rate (MFR) of about 2 units.
Organic peroxides are widely used with PP and PP/PE blends for visbreaking. Visbreaking involves heating the PP or mixture of PP with a peroxide in the melt phase in an extruder or similar mixing device capable of imparting a lower molecular weight product and, accordingly, increasing the MFR. In some instances, visbroken polymers can be produced without the use of peroxides by thermal cracking. Procedures for visbreaking various propylene polymer compositions are disclosed in U.S. Pat. Nos. 3,607,987; 4,375,531; 4,508,872; 5,066,723 and 5,218,046.
U.S. Pat. No. 5,639,818, while recognizing that polymers or polymer blends which are visbroken by conventional procedures, do not contain long-chain branching and accordingly are deficient in melt strength, discloses a multi-step process whereby PP/PE blends having improved melt strength are produced. The process comprises adding a peroxide in an amount ranging from about 100 to about 1000 ppm to a non-crosslinked LDPE or an ethylene copolymer containing a vinyl acetate, a methyl acrylate, a n-butyl acrylate, an α,ω-diene or an unsaturated alkoxysilane for a period of time to produce a peroxide-adsorbed component and melt mixing the peroxide-adsorbed polymeric component with a polypropylene homopolymer at a temperature of about 170° C. to about 300° C. wherein the polymeric blend comprises about 5 to about 40 weight percent of the non-crosslinked polymeric component and about 95 to about 60 weight percent of the polypropylene homopolymer.
It would be highly advantageous if propylene polymers could be modified using organic peroxides to produce long-chain branching and improve melt strength. It would be even more advantageous if this could be accomplished using low levels of peroxide and in the solid state. These and other advantages are achieved with the improved process of the present invention.